Development of aFusarium oxysporumf. sp.melonisfunctional GFP fluorescence tool to assist melon resistance breeding programmes

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Fusarium wilt, caused by Fusarium oxysporum f. sp. melonis (Fom), is one of the most widespread and devastating melon diseases. This vascular disease is caused by the colonization of melon xylem vessels by any of the four Fom races reported (r0, r1, r2 and r1,2, subdivided into r1,2w and r1,2y). The macroscopic evaluation of disease symptoms (disease rating, DR) at several days post-inoculation (dpi) with Fom spores has been the traditional method to determine the resistance of melon accessions to this fungal pathogen. In this study, one isolate from each Fom race was transformed by Agrobacterium tumefaciens to constitutively express the green fluorescent protein (GFP). Fom-GFP transformants, as virulent as the corresponding wildtype races, were selected to develop an inoculation assay based on the non-invasive evaluation of the fluorescence emitted by Fom-GFP. It was determined that melon root neck was the appropriate area to follow Fom-GFP and a fluorescence signal rating (FSR) was established in parallel to DR determination. This method allowed the evaluation of GFP signal in the root neck of inoculated melon seedlings at 11–15 dpi. The GFP signal was scored in 62 melon accessions/breeding lines inoculated with different Fom-GFP, followed by evaluation of the macroscopic DR in the aerial part of melon seedlings at 20–28 dpi. Correlation analysis demonstrated a direct and significant relationship between FSR and DR. This method has shown to be an effective and reliable tool that can assist Fom resistance breeding programmes in melon.

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